We start by creating a circle. Select the Ellipse Shape tool (L). Hold down Shift key while dragging. Turn on Smart Guides, go to View > Smart Guides or hit command/control + u. Make sure your Rulers are turned on. Go to View > Show Rulers or hit command/control + r. Drag a horizontal guide from the top Ruler to the middle of the circle. Select the Rotation tool and click in the center of the circle to set the rotation point. Rotate the circle 45°. While dragging hold down Shift key so the rotation snaps to 45°. With Smart Guides turned on you should also be able to see when you've turned the circle 45°. Select the Direct Selection tool (white arrow) and select the path segment on the right of the circle. Hit the delete key. ...............